is a series of papers in which no simple relationship was shown to exist between endogenous ABA and
leaf [105,106], fruit, or flower bud [107–109] abscission. However, correlations cannot prove or disprove
involvement, and it could be argued that increases in tissue sensitivity to the hormone could induce ab-
scission without a change in ABA concentration. Another general problem encountered when measuring
hormone levels is that the value obtained represents an overall mean concentration for the piece of mate-
rial and ignores important local variations. For instance, a very thin (2 mm) slice made to remove the AZ
from a petiole will at best contain only 10% abscission zone cells, and the hormone concentrations in
these could be very different from those in the adjacent, contaminating senescing tissue.
In a review of ABA action, Milborrow [110] expressed surprise that among the hundreds of plants
sprayed with ABA, more did not show an abscission response. By way of a reply, Addicott [55] listed a
considerable number of cases where a response does occur. Abscisic acid is much more effective when
applied to explants, and some scientists believe that it acts by promoting ethylene formation. For instance,
Sagee et al. [111] have reported that ABA is ineffective in the presence of the ethylene synthesis inhibitor
AVG. The current consensus seems to be that ABA is not directly involved in abscission. However, there
is some evidence that it may be important in cereal seed shedding, which is one of the few ethylene-in-
sensitive systems [112].
216 SEXTONFigure 9 Modern representation of the ethylene auxin balance theory. The relative concentrations of auxin
and ethylene in the abscission zone are represented by the weight of beakers on either side of a balance. If the
left-hand side goes down, the induction of cell separation occurs and abscission takes place. The weight of the
containers is influenced predominately by supply from the distal organ, such as a leaf or fruit. Some factors that
influence this supply are shown. The balance can also be tipped in either direction by moving the fulcrum from
side to side. This represents changes in the tissue’s sensitivity to either hormone. Moving the fulcrum to the
right increases sensitivity to ethylene, and some factors that do this, such as water stress and aging, are shown.
Moving the fulcrum to the left increases sensitivity to auxin and decreases ethylene responsiveness. Adding
auxin itself or cytokinin will cause this to occur. (From R. Sexton, unpublished.)